This invention relates to thickness and lateral position measurement having particular application to transparent webs in roll to roll manufacturing processes.
Roll to roll manufacturing processes provide high throughput production at economical rates. These processes require accurate positioning an handling of large sheets or “webs” that must be properly unwound, processed, and rewound in a continuous fashion. The accuracy in which the web can be positioned limits the intricacy and complexity of the features able to be manufactured via roll to roll processing. The sensors that measure the properties of the web face additional complications when the webs are made from transparent materials. By improving the sensing capabilities of tools that measure transparent webs, the economic benefit of roll to roll manufacturing processes can be used in a wider variety of applications.
Roll to roll manufacturing hardware must measure and control the position of the web perpendicular to the direction of travel. The precision in which the lateral position of the web can be measured and controlled affects the performance of the roll to roll process. For example, the density of data stored on magnetic tapes is chiefly limited by the fidelity of the spatial registration between the ribbon and the read head [1]. The most common measurement strategy used in industry will sandwich the web between a transmitter and a receiver. As the web moves further into the range of the sensor the received signal becomes attenuated as the transmitted signal becomes blocked. Sensors measuring opaque webs typically use light as the transmitted signal [2-3] while those measuring transparent webs will use sound [4-5]. Due to the greater diffraction of sound waves, ultrasonic sensors exhibit resolution that is poorer than the light based sensors. Seshardi and Pagilla have demonstrated a sensor to measure both mediums [6].
In a variety of industrial roll to roll applications such as coating or extrusion processes it is important to inspect and control the thickness of the web. One prevalent method to measure thickness and index of refraction of transparent plates uses lateral shear interferometery to pass a spatially coherent laser multiple times through a transparent object in order to observe the subsequent interference pattern [7-9]. Other methods observe light reflect from the front and back surface of the transparent object to determine is thickness [10-12]. Although many of these methods are able to simultaneously measure both thickness and index of refraction, it is also common and easier to instead measure their product: the optical path length. With more accurate measurement of thickness the products made by roll to roll processes can be both more intricate and cost effective.
An object of this invention is a sensor that can simultaneously measure a transparent object's optical path length and lateral position with submicron resolution.